Increasing the fatigue limit of high-strength steels (100Cr6 and 42CrMo4) by thermomechanical treatments

M.Sc. Amin Khayatzadeh

It has already been shown that a suitable thermomechanical treatment (TMT) in the temperature range of maximum dynamic strain aging (DSA) can increase the fatigue strength of high-strength steels for high-cycle fatigue (HCF).
In the field of Very High Cycle Fatigue (VHCF), the failure of high-strength steels typically starts from critical inclusions in the material volume.
It might be possible for these materials, to strengthen the microstructure of critical inclusions by finding an appropriate thermomechanical treatment at maximum DSA range.

It should be understood how a TMT affects the microstructure as well as the lifetime for very high cycle fatigue.
A suitable TMT should be found to optimize fatigue strength in the VHCF range.
A model should be developed to explain the relation between the microstructure and HCF and VHCF lifetime

For the materials 42CrMo4 and 100Cr6, the temperature with the maximum DSA is identified for a given stress amplitude.
At the temperature of maximum DSA, some load cycles with increasing stress amplitude are applied to strengthen the microstructure.
HCF (Nf ≤ 10⁷) and VHCF (Nf ≥ 10⁷) experiments need to be performed on the samples.
Fractography using SEM and TEM analysis of the microstructure to identify failure mechanisms and microstructure around crack initiation sites: Does TMT treatment affect microstructure and failure mechanisms?

Technical University of Kaiserslautern